Transforming meadows into free surface water wetlands: Impact of increased nitrate and carbon loading on greenhouse gas production

Abstract

In a laboratory study we investigated 1) the potential production of nitrous oxide (N 2O), methane (CH 4) and carbon dioxide (CO 2) and 2) the effect of nitrate (NO 3 −) and anaerobic N 2O development on CH 4 production in sediment from a recently recreated free surface water wetland (FSWW) and in soil from an adjacent meadow. We designed an experiment where production of greenhouse gases was registered at the time of maximum net development of N 2O. We made additions of biodegradable carbon (glucose) and/or NO 3 − to sediment and soil slurries and incubated them at four temperatures (4, 13, 20, 28 °C). Gas production from both substrates was positively correlated with temperature. We also found that the sediment produced more N 2O than the soil. N 2O production in sediment was NO 3 − limited, whereas in soil carbon availability was lower and only combined additions of NO 3 − and glucose supported increased N 2O development. CH 4 production was generally low and did not differ between soil and sediment. Nor did glucose addition increase CH 4 rates. The results suggest that neither soil nor sediment environment did support development of methanogenic populations. There were no clear effects of NO 3 − on CH 4 production. However, the highest records of CH 4 were found in incubations with low N 2O production, which indicates that N 2O might be toxic to methanogens. In summary, our study showed that transforming meadows into FSWWs implies a risk of increased N 2O emissions. This does not seem to be valid for CH 4. However, since N 2O is almost always produced wherever NO 3 − is denitrified, increased N 2O production in wetlands leads to reduced rates in downstream environments. Hence, we conclude that when balancing NO 3 − retention and global warming aspects, we find no reason to discourage future creation or restoration of wetlands.